In recent year, a variety of portable electronic apparatuses represented by a notebook personal computer have been structured into cordless and portable forms. Thus, a variety of small thickness, compact size and light weight portable electronic apparatuses have sequentially been developed. Since the apparatuses have considerably varied, the quantity of electric power which is required for the apparatuses has been enlarged. Hence it follows that batteries which are energy sources for the apparatuses, in particular, secondary batteries are required to have larger capacities.
As conventional secondary batteries, lead batteries and nickel-cadmium batteries have been known. Moreover, new secondary batteries, such as nickel-hydrogen batteries and lithium-ion batteries, have been put into practical use. The foregoing secondary batteries each of which incorporates solution to serve as the electrolyte thereof, however, suffer from a problem of leakage of liquid.
To solve the above-mentioned problem, a polymer lithium-ion secondary battery incorporating polymer gel swelled by the electrolytic solution and serving as the electrolyte has been developed. Since the polymer lithium-ion battery has been developed, the problem of the leakage of the solution from the battery can be prevented. Thus, a small size, light weight, small thickness and high energy-density secondary battery can be realized.
The structure of the polymer lithium-ion secondary battery will now be described. A collector for the positive electrode made of a thin aluminum plate incorporates active materials composed of LiCoO2 and graphite and laminated thereon. Moreover, active materials composed of carbon, cokes, graphite and so forth are laminated on the collector for the negative electrode made of a thin copper plate. Therefore, the electrodes are constituted. Moreover, a separator made of polypropylene or polyethylene and structured into a thin film having pores is sandwiched between the electrodes. In addition, a polymer gel electrolyte, such as polyacrylonitrile (PAN), polyethylene oxide (PEO) or polyvinylidene fluoride (PVDF), is enclosed in spaces among the electrodes and the separator. Thus, the polymer lithium-ion secondary battery has a sandwich structure.
The unit cell having the sandwich structure is packaged into a case serving as an encapsulating container and constituted by a thin metal film, such as aluminum foil, and a plastic film made of nylon, polyethylene, polypropylene or polyethylene terephthalate.
When the aluminum laminate pack is employed to serve as the case for the lithium-ion battery, reduction in the weight and thickness can be realized. A larger quantity of water, however, introduced into the aluminum laminate pack as compared with the conventional metal container (a battery can). Hence it follows that lithium ions are consumed owing to decomposition of water and decomposed gases and hydrofluoric acid are produced. Therefore, there arises a problem in that the capacity of the battery is reduced excessively.
When water in a quantity of 350 μg or greater per capacity (Wh) of the battery is introduced, the capacity is reduced to 80% or smaller. Therefore, a requirement has arisen for a battery package free from reduction in the capacity, that is, with which the quantity of water which will undesirably be introduced into the battery package can be reduced to a value which is not larger than the above-mentioned value.